My Honda Civic has a D gear and a D3, which apparently prevents it from going into 4th. Why would I want to do this? What is it good for? Does it make my fuel efficiency better or worse? Does it make the transmission's job easier or wear it out faster?

5 Answers
5

This is analagous to down shifting in a manual. This is a lower gear for the transmission which means the engine revolves at a higher rate producing more back pressure at the same speed as a higher gear.

When going down a hill, if you downshift that will reduce the demands on the braking system, due to the back pressure. You will often see truck drivers downshifting on long hills so that their brakes do not overheat.

You would only use D3 while going downhill with a load so you can use your brakes less. Your mileage would be worse since the engine is running at a higher rpm. Only if you did a lot of using D3 would it make any appreciable wear on your transmission.

If by "mileage" you mean your fuel consumption, down-shifting on a hill will likely have little if any MPG difference because the engine is likely closed throttle much if not all of the time. My manual car with instantaneous MPG gauge, and the difference going down hill between 5 and 3 is negligible.
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Sean ReifschneiderMar 8 '11 at 5:48

@Sean, on steeper grades, when throttle is closed, most FI systems shut off fuel altogether in the condition of over-run (higher RPM, closed throttle), so if car runs away from you, down-shifting and engine-braking is better fuel-economy wise.
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theUgMay 28 '12 at 22:18

@theUg: This is interesting conjecture, but I still stand by my statement that there will be little if any change. If it completely shuts down the fuel supply, that's likely not much of a difference from the very, very little amount injected if it doesn't. Certainly in my car, it says "MPG is off the charts high" in a higher gear coasting downhill.
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Sean ReifschneiderMay 29 '12 at 21:58

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@SeanReifschneider is probably right that the difference is small enough to be hard to measure and possibly insignificant to normal people. The actual fuel flow is easily measured with an OBD-II reader, though: that should be cited as the true answer to any specific situational fuel consumption question for a particular car.
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Bob Cross♦Aug 7 '12 at 19:57

In addition to what Patrick said, D3, or whatever else it's called in various makes and models, is also useful for those times when you're climbing a hill and the transmission keeps shifting back and forth between gears.

There may also be times, such as when driving on very slippery surfaces, when a gear shift could cause the car to lose traction. In such a situation the ability to hold the car in a given gear can be very advantageous.

Good explanation, but in situations like that (bar the unsafe one you described in the second paragraph) I prefer to either reduce cruise control speed, or, better, altogether turn it off and let the car lose a little speed to keep it in the same gear. Or, better yet, drive a manual. :)
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theUgMay 28 '12 at 22:13

Personally I use D3 in and around town, in built up areas (<50km/h or 30 mph) there is no need for D4. Use D4 on the open road (motorway/freeway) for speeds above 60km/h (>40 mph). Also use D3 for towing and hill climbing/descending for better engine braking and avoiding constant shifting between D3 and top. Worked mint for me (still very economical) and I have owned 3 Accords :D

Is there any harm in shifting between them while moving?
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endolithAug 7 '12 at 15:46

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@endolith, no, shifting between D3 and D4 using the manual control is exactly the same as when the automatic handles it on its own. By moving the lever D4->D3, you're telling the transmission "please don't use fourth gear." Moving D3->D4, you're saying "go ahead and use fourth gear if you need to."
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Bob Cross♦Aug 7 '12 at 19:54

I think using D3 in areas that are signed 40MPH or less eliminates a lot of shift changes between the various higher speeds on your transmission. If the road is signed 40 MPH and under, I always use the D3 button. I also use it if there is ice or snow on the roadway, but the main thing is the wear and tear on a transmission trying to go up a hill or around a curve in a very high gear which is hard on engine and transmission.